At the present time, our laboratory is isolating both the cyanogen bromide fragments and the methionine-containing tryptic peptides of the large chain of (Na ion plus K ion) ATPase. Partial amino acid sequence information from these peptides will allow us to order the cyanogen bromide fragments to form a linear map of this protein. This procedure will divide the primary structure of the large chain into 28 consecutive regions. We are also devising a two-dimensional gel electrophoresis system in order to separate rapidly all of the fragments from a digest. With this procedure it will be possible to determine in which of these 28 regions of the sequence a particular amino acid, which has been labeled covalently in some fashion, is located. By labeling inside-out and right-side-out vesicles containing (Na ion plus K ion) ATPase with impermeable reagents, it will be possible to determine which regions of the linear sequence of the protein contain amino acids exposed to the external medium, and the cytoplasm. Cross-linking experiments will yield information about the proximity of different segments of the protein in its native structure. Affinity labels can be synthesized which bind to specific ligand sites and covalently label amino acid residues in the vicinity of these sites. Each of the results of such experiments will either position a region of the protein within one of three phases, or cluster certain sequence regions around one of the cross-linking sites or ligand sites. As more and more information is gathered, it will be possible to coil the protein into a rough approximation of its three-dimensional structure.